The present invention relates to an apparatus for defrostable ventilation systems which include an element for the transfer of heat from warm exhaust air (taken from inside a building) to cooler exterior fresh air (drawn into the building), the system using warm interior air as defrost air for defrosting the system during cool weather.
Heat recovery ventilation systems are known whose function is to draw fresh exterior air into a building and to exhaust stale interior air to the outside. The systems are provided with appropriate ducting, channels and the like which define a fresh air path and an exhaust air path whereby with the interior air of a building and the exterior ambient air may be exchanged; during ventilation the air in one path is not normally allowed to mix with the air in the other path. A heat recovery ventilator device which forms part of such a system is provided with corresponding air paths and also one or more heat exchange elements or cores.
Heat recovery ventilation devices may also have a housing or cabinet; such enclosures may for example be of sheet metal construction (e.g. the top, bottom, side walls and any door, etc. may be made from panels of sheet metal). The heat exchanging element(s), as well as other elements of the device such as, for example, channels or ducts which define air paths, filtration means, insolation and if desired one or more fans for moving air through the fresh air and exhaust air paths may be disposed in the enclosure. Such ventilation devices may be disposed on the outside of or within a building such as a house, commercial building or the like; appropriate insulation may be provided around any duct work needed to connect the device to the fresh air source and the interior air of the building.
A problem with ventilation equipment used to extract heat from exhaust air, is the production of frost or ice in the exhaust path of the system. During cold weather, prior to expelling the relatively warm exhaust air, the equipment provides for the transfer of heat from the relatively warm exhaust air to the relatively cool (fresh) outside air by the use of a suitable heat exchange element. However, since the warm interior air will usually contain a certain amount of moisture, the cooling of the interior air can result in the formation not only of water but of ice if the exterior air is below the freezing point of water. An uncontrolled buildup of ice on the exhaust air side of the system can result in decreased heat transfer, and even outright blockage of the exhaust air path. Accordingly a means of periodically defrosting such a system is a requirement in order to maintain it's efficiency.
A defrost mechanism is, for example, proposed in U.S. Pat. No. 5,002,118. In accordance with the teachings of this patent the fresh air intake is periodically blocked off by a damper and warm interior air is injected, via a separate defrost air conduit, into the fresh air inlet side of the fresh air path of the ventilation apparatus. However, during the defrost cycle of this known system, stale inside air is still exhausted to the outside via the exhaust air path; this is disadvantageous since by blocking only the fresh air inlet and continuing to exhaust interior air to the outside, a negative air pressure can be built up in the interior of a building relative to the exterior atmosphere. Such a negative pressure may induce uncontrolled entry of air through any cracks and cranies in the structure of the building; the negative pressure may, in particular, produce a backdraft effect, for oil and gas type heating systems, whereby exterior air may be pulled into the chimney leading to the accumulation of gaseous combustion products in the building. The above mentioned patent does not address this type of problem.
It would therefor be advantageous to have a system which can use interior air as defrost air while diminishing or avoiding the creation of a negative air pressure in the building.
It would also be advantageous to have a defrostable ventilation apparatus of the kind described herewithin which is of simple construction.